1. Field of the Invention
This invention relates generally to voltage regulating systems employing tap-changing transformers, and more specifically, this invention relates to a fail-safe circuit for verifying that the non-arcing switch in the tap-changing transformer system is operating properly before a movable tap contact is moved to change the tap setting of the transformer.
2. Description of the Prior Art
In order to prevent the undesired, and sometimes destructive, effects of line voltage variations, various types of voltage regulators have been developed. One type of voltage regulator is the tap-changing transformer. This type of regulator is particularly useful in connection with the regulation of relatively high line voltages for the provision of relatively high power requirements.
A tap-changing transformer voltage regulating system employs a shunt winding, connected from the load line to the common side of the source, or directly across the source, and a series winding connected from the load line to the high or energy side of the source. The series winding is tapped or divided into a plurality of sections, with a stationary contact associated with each division between sections. A movable tap contact (or tap contacts) is automatically driven in response to a change in the line voltage. A reversing switch is automatically actuated in order to cause the flux produced by the series winding to be either additive or subtractive with respect to the flux of the shunt winding, hence increasing or decreasing the line voltage as required.
Inasmuch as the load current is passing through the series winding, a tap change can result in the formation of relatively large electrical arcs. Such arcs are destructive to the movable tap contacts and the stationary contacts, thus reducing the life of the tap switch. In addition, since the tap-changing transformer is located in an oil bath, the action of the arc on the oil causes reduction of the oil to produce carbon in a solid or particulate form and hydrogen in gaseous form.
In order to minimize undesirable arcs at the tap switch, a pair of tap contacts are individually connected to the source through a line having a bridging reactor (or preventive autotransformer). A non-arcing switch is connected between the parallel tap contact lines, and each of the lines includes a current redirecting switch on the source side of the non-arcing switch. When it is desired to move a tap contact, a control motor is energized to open the current redirecting switch in that line to cause the current flow for that tap contact to pass through the normally closed non-arcing switch. The non-arcing switch is then opened to break the current flow to that tap contact, after which the control motor transfers the tap contact. The non-arcing switch is then closed, after which the current redirecting switch for the tap contact that was moved is closed.
While this arrangement works quite well, failure or improper operation of the non-arcing switch, or failure of a current redirecting switch to open, will result in production of undesired arcs at the tap-changing transformer contacts. Such failures, and the resultant undesired arcs, could go undetected for quite some time, thus resulting in considerable contact damage and the reduction of oil. Accordingly, it is desirable to remove the tap-changing transformer regulating system from the load circuit and/or indicate an improper operation of the current redirecting or non-arcing switches.
In general, the prior art reveals little in the way of concrete efforts to overcome the problems of a failure in the current redirecting and non-arcing switches. An example of one approach that has been tried is illustrated in U.S. Pat. No. 3,436,646--Prescott. This arrangement utilizes a number of switches to test for circuit conditions after each desired opening or closing of a current redirecting switch or the non-arcing switch. Hence, as shown in FIG. 2 of the patent, a sequential checking of the current redirecting and transfer switches appears to be achieved. While this reference at least shows an attempt to deal with the problem of a failure in a current redirecting and non-arcing switches, it involves a rather complex arrangement that provides more information than is actually required, so long as the protecting system is operating. Accordingly, the prior art does not disclose any simple, reliable arrangement for determining that a tap-change can be effected without fear of an undesirable arcing condition.